Prior commercial processes for removing sugar from sugar cane may be classified generally as tandem mill processes or diffusion processes. In tandem mill processes, the fiberized cane is repeatedly subjected to high pressures, usually in the range of about 1,000 lbs./sq. inch, in order to press the sugar juice from the cane. In diffusion processes, solvent is allowed to percolate through a bed of fiberized cane by gravitational flow for dissolving and extracting the soluble sugar. Many modifications of the foregoing processes, of course, have been made; for example, imbibition liquid ordinarily is utilized in tandem mill processes to improve the extraction efficiencies.
In the past, improvements in tandem mill processes have been in the direction of creating apparatus for exerting higher and higher pressures on the cane in an attempt to increase the extraction efficiencies. This has resulted in higher and higher power requirements, accompanied by increased maintenance costs, and has resulted in increased operational expenses.
The high pressures employed in the conventional tandem mill allows only a small maceration rate. This is because the large juice extraction resulting from the high pressures restricts the amount of maceration. If higher maceration rates could be possible in such tandem mills, the poor drainage area of conventional three-roll mill would be restrictive. A moisture content in bagasse above about 70 percent by weight creates severe feeding problems. Many devices have been developed in an attempt to improve the feeding of such wet bagasse, but with only limited success. In tandem mill processes, the use of high pressures and force feeding devices, therefore, necessarily make such equipment applicable only to materials having a relatively low moisture content.
Sugar cane harvesting methods now are tending toward the incresed use of mechanical equipment. Mechanical harvesting equipment, however, introduces more foreign material into the extraction process, causing increased wear and also increasing the maintenance costs in the high pressure mills.
The diffusion processes require comparatively more elaborate, complex, and expensive equipment than the tandem mill processes, and, therefore, they are economically most feasible in factories having high manufacturing capacities. The extraction efficiencies in the diffusion processes can be higher than for all other available types of extraction equipment, and, for this reason, can be economically justified in many operations. The cane, however, must be specially prepared and exact temperature controls are necessary to prevent adverse bacteriological changes in the sugar during the timer period required for the diffusion operation.